A primary requirement for toughness in ceramic composites is the existence of a weak interface (or interphase) between constituents of the composite, such as between matrix and reinforcement materials. A weakly bonded interface allows sliding between the reinforcements and the matrix and/or preferential crack deflection around the reinforcements for optimal toughening of the composite. In fibrous composites the weak interface allows the matrix to crack and/or deform without damaging the fibers. In particulate composites clouds of microcracks can form around a large crack and disperse the rupture process. In multilayered composites the individual layers can fracture independently and disperse the rupture event to produce a non-catastrophic response.
An ideal interface between a reinforcement and a ceramic matrix must be sufficiently weak to allow debonding and sliding of the reinforcement when a crack impinges upon it from the matrix. If this does not occur, the crack passes through the reinforcement with minimal or no toughening of the composite. A relevant property of the interface is the debond energy, .GAMMA..sub.i, of either the interphase material or the actual interfaces between the reinforcement, interphase material, and matrix. The debonding criterion is generally satisfied if .GAMMA..sub.i /.GAMMA..sub.f .ltoreq.0.25, where .GAMMA..sub.f is the fracture energy of the reinforcement.
Ceramic composites are desirable in certain applications because of their refractory properties. For a high temperature composites, however, further requirements are imposed on the weak bond material: it must be weak and stable over the entire temperature range of use, chemically compatible with the other materials of the composite, and morphologically and environmentally stable at high temperatures. Existing fibrous and multilayered ceramic composites rely on carbon, boron nitride, or micaceous materials (e.g., fluorophlogopite) to provide the weak interface. Examples of these composites include various glasses, glass ceramics, silicon carbide, and silicon nitride reinforced with SiC or Al.sub.2 O.sub.3 fibers; alumina, silicon nitride, or MoSi.sub.2 reinforced with SiC whiskers; and multilayered laminates having layers of SiC and carbon. At higher temperatures, however, carbon and boron nitride interphase materials oxidize readily and micaceous materials react with reinforcement and matrix materials.
Machinable glass ceramics are another example of ceramic composites that rely on easy debonding. These composites contain platelets of a mica, such as fluorophlogopite, that cleave easily and cause chipping when the surface is contacted by a hard point. Because of this easy chipping, the material can be shaped using conventional metal working processes such as milling, drilling, and turning that remove material at a single contact site (rather than the more expensive and less versatile multipoint grinding that is needed for most ceramics).
Composites containing layers of interface materials selected from the .beta.-alumina/magnetoplumbite family of structurally related materials have been developed for use in high temperature, oxidizing environments. These materials are described in U.S. Pat. No. 5,137,852 issued to Morgan et al., the teachings of which are incorporated herein by reference. Experimental work with these materials has shown, however, that it is difficult to find suitable composite systems comprising a ceramic matrix; reinforcements having high strength and high Young's modulus; and a weakly bonded interface material that is morphologically stable in high temperature oxidizing environments, chemically compatible with the matrix and fiber system, and a good match to the thermal expansion of the matrix and fibers. Because most suitable reinforcements and matrices are multiphase materials, the compatibility of the materials is reduced, particularly over a range of temperatures, and the complexity of chemical processing is increased. Thus, there is a need for high temperature ceramic composites that are less complex, have a weakly bonded interface between reinforcement and matrix materials, and are morphologically stable in high temperature reactive environments.